Magnetic data storage media, such as magnetic tape and magnetic disks, are commonly used for storage and retrieval of data. Magnetic data storage media can be categorized as longitudinal or perpendicular. Most conventional magnetic media are longitudinal. In longitudinal media, the largest magnetic remanence may be obtained in a direction generally parallel to the plane of the medium. In other words, in longitudinal media, the magnetic preferred-axis orientation of individual magnetic domains is close to parallel to, or in-line with, the surface of the medium and the direction of medium travel. In perpendicular media, on the other hand, the largest remanence magnetization of the magnetic particles is possible perpendicular to the plane of the medium. In other words, in perpendicular media, the magnetic preferred-axis orientation of individual magnetic domains is predominately perpendicular to the medium surface. However, magnetic media may have particles capable of both a significant longitudinal and significant perpendicular components in its orientation. Perpendicular media generally allow for a much higher storage density than can be achieved in longitudinal media.
The magnetic storage media generally have a series of transitions between different magnetized regions. The different magnetized regions may encode a series of bits that represent a value of “0” or “1”. The magnetically oriented regions may be aligned on data tracks that divide the magnetic medium. In order to subsequently store data on the magnetic storage medium, a recording head of a magnetic drive, such as a tape drive or disk drive, encodes data by selectively orienting various magnetic regions on the medium. A read head, or transducer head, of a magnetic drive may later be positioned relative to the data tracks to detect the regions, and the drive can interpret the detected regions to retrieve the data.
During data storage and recovery, the head must locate each data track, and follow the path of the data track accurately along the media surface. In order to facilitate precise positioning of the transducer head relative to the data tracks, servo techniques have been developed. Servo patterns refer to signals or other recorded marks on the medium that are used for tracking purposes. In other words, servo patterns are recorded on the medium to provide reference points relative to the data tracks. A servo read head has a fixed displacement relative to the transducer head that reads the data tracks. The servo read head can read the servo patterns, and a servo controller interprets a detected servo pattern and generates a position error signal (PES). The PES is used to adjust the lateral distance of the servo read head relative to the servo pattern and the transducer head relative to the data tracks so that the transducer head is properly positioned along the data tracks for effective reading and/or writing of data to the data tracks.
With some data storage media, such as magnetic tape, the servo patterns are stored in specialized tracks on the medium, called “servo bands.” Servo bands serve as references for the servo controller. A plurality of servo patterns may be defined in a servo band. Some magnetic media include a plurality of servo bands, with data tracks being located between the servo bands.
One type of servo pattern is a time-based servo pattern. Time-based servo techniques refer to servo techniques that make use of non-parallel servo marks and time variables or distance variables to identify head position. The time offset between the detection of two or more servo marks can be translated into a PES, which defines a lateral distance of the transducer head relative to a data track. For example, given a constant velocity of magnetic tape formed with servo pattern “/ \”, the time between detection of mark “/” and mark “\” becomes longer when the read head is positioned towards the bottom of pattern “/ \” and shorter if the read head positioned towards the top of pattern “/ \”. Given a constant velocity of magnetic media, a defined time period between detected servo signals may correspond to a center of pattern “/ \”. By locating the center of pattern “/ \”, a known distance between the center of the servo band and the data tracks can be identified. Time-based servo patterns are also commonly implemented in magnetic tape media, but may be useful in other media.